University of Maryland Extension

Watering and Soluble Salts - Houseplants

A large percentage of houseplants are lost because of overwatering and underwatering. Watering on a schedule is not the best method as this can lead to plants receiving too much or too little water. One way to determine when to water is to test the soil with your finger to a depth of about two inches. If the soil is dry, it probably needs to be watered. This method works for most plants but there are exceptions like succulents and cacti which need far less water. Certain plants like peace lily (Spathiphyllum) will wilt when they need to be watered although plants can also wilt if they are overwatered too. Another quick test is to lift the plant, pot and all, to check its weight change. The plant with dry potting medium will weigh much less than the plant which still has ample water in its potting medium. While you’re testing for watering needs, pay attention to the potting media. If your finger can’t penetrate two inches deep, you may need a more porous potting mix or the plant may be root bound.

There are a couple of ways to water; watering from the top or watering from the bottom. Top watering is the most common method. Irrigate so that the water drains freely from the bottom of the container. Small plants can be placed in the sink and returned to their locations after the water drains. Any excess water should be dumped from drainage saucers. Bottom watering simply means putting water into the saucer or setting the container in a pot of water allowing the water to be absorbed and drawn up into the potting mixture. Empty out any excess water. Never let your houseplants sit in water. 

Soluble Salts Buildup

A problem of great importance to houseplants is the buildup of soluble salts in the potting medium, which can easily be avoided with proper watering techniques. Symptoms of this buildup include reduced growth, brown leaf tips, dropping of lower leaves, dead root tips, and wilting. In addition, a ring of salt deposits may crust around the pot at the soil line, around the drainage hole, or on the exteriors of clay pots.

Soluble Salts

• Are minerals (fertilizers) dissolved in water;
• Stay behind and become concentrated when water evaporates from the mix; and
• Can inhibit the plant’s uptake of water.

If salts build up to an extremely high level in the soil, water is drawn out from the root tips, damaging the roots directly. The weakened plant is more susceptible to attack from insects and diseases. All water contains dissolved minerals, but softened water contains more than others and should be avoided when watering houseplants.

To Prevent Soluble Salts Injury

• Water the pot from the top until water runs out the bottom, washing out all the excess salts (fertilizer residue);
• Don’t let the pot sit in the water that runs out; and
• After watering, wait until the potting medium dries at the two-inch depth before watering again.

To prevent mineral buildup, water with clear water to leach the soil of your houseplants every four to six months. Leach by performing these simple steps:

1. Before you begin leaching, manually remove the salt crust if a layer of salts has formed on top of the potting medium;
2. Pour a lot of water -- at least twice the volume of the pot -- on the potting soil and let it drain completely;
3. Keep the water running through the potting soil to wash the salts out;
4. Leach a plant before you fertilize so that you don’t wash away all the fertilizer you just added; and
5. Repot the plant if the soluble salts level seems high or the pot has poor drainage.

The level of salts injurious to the plant varies with the type of plant and how it’s grown. A plant grown in a greenhouse with excellent light and drainage will tolerate salt levels 10 times higher than one grown in your home. Some nurseries and plant shops leach plants before sale to remove excess salts. It’s a good practice to leach every newly purchased plant the first time you water.

 Watering Plants without Mineral Buildup

Prevent salt build-up by choosing mineral-free water. The next time rain is expected, set some buckets outside and let them fill with rainwater, which has no added chemicals. If you use a dehumidifier, use the water in the drain pan, which is distilled water, on your plants. Otherwise, draw water for your plants the day before you plan to water them. A day’s evaporation should clear the water of chlorine and give enough time for most minerals to settle harmlessly to the bottom. Discard the bottom one to two inches of water.

 

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